Trimethyllysine (TML) is the putative precursor of carnitine in humans, but little direct experimental evidence exists for this conclusion. Understanding carnitine metabolism is important because it plays a major role in mitochondrial transport and oxidation of long chain fatty acids. Carnitine deficiency has life threatening consequences in infants if it is not detected and treated. Dietary supplementation provides an easy means to overcome the deficiency. Still, without clear understanding of the biosynthetic pathway in infants, the origin of carnitine deficiency cannot be fully understood. In this project, the putative carnitine precursors, TML, was administered to preterm infants, and carnitine levels in urine and plasma were analyzed by FAB-MS after esterification of the carboxyl group to enhance sensitivity. We also searched for the accumulation of possible intermediates, such as hydroxy-TML. The results showed that both TML and carnitine were present in higher concentration in urine and plasma after TML administration, but no intermediates were detected.